Aggregation Kinetics and
Transport of Single-Walled
Carbon Nanotubes at Low Surfactant Concentrations
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Abstract
Little is known about how low levels of surfactants can
affect
the colloidal stability of single-walled carbon nanotubes (SWNTs)
and how surfactant-wrapping of SWNTs can impact ecological exposures
in aqueous systems. In this study, SWNTs were suspended in water with
sodium dodecylsulfate (SDS) as a surface-active dispersing agent.
The effect of SDS concentration on SWNT suspension stability was investigated
with time-resolved dynamic light scattering (TRDLS) initial aggregation
studies utilizing both monovalent (Na<sup>+</sup>) and divalent (Ca<sup>2+</sup>) cations. The critical coagulation concentration (CCC) values
increased with SDS concentration for the Na<sup>+</sup> treatments,
but the Ca<sup>2+</sup> treatments were less sensitive to SDS concentration
changes. Longer term stability studies with SDS concentrations orders
of magnitude below the SDS critical micelle concentration demonstrated
that SWNTs remained suspended for over six weeks in a surface water.
Transport studies in a freshwater sediment similarly showed a SDS
concentration-dependent mobility of SDS–wrapped SWNTs in that
SWNTs showed a relatively greater retention at lower SDS concentrations
(0.001%–0.05% w/v) than at a higher SDS concentration (0.1%).
It is hypothesized that the stability and mobility of SWNT suspensions
is directly related to the surface coverage of SDS on the SWNT surface
that simultaneously increases electrosteric repulsion and decreases
surface chemical heterogeneity. Overall, these studies demonstrate
that low levels of surfactant are effective in stabilizing and mobilizing
SWNTs in environmental media